Three piece trucks, which are comprised of two parallel sideframes and a bolster extending therebetween, are well known and used within the majority of freight railcars in service today. Each sideframe is comprised of a upper compression member, a lower tension member, and a pair of vertically extending support columns which join the upper and lower members together. The upper compression member has a pair of ends, each of which includes a pedestal jaw depending therefrom for receiving the transversely extending wheel axles. The lower tension member extends in a generally parallel direction to the upper member and is comprised of a longitudinal central portion which also has a pair of ends. Each end is comprised of an upwardly extending diagonal arm which extends to and attaches with the upper compression member and pedestal jaw. The vertical support columns in each of the sideframes are longitudinally spaced from each other and attach to the lower tension member where the lower member ends upwardly extend, thereby forming the bolsters opening in their respective sideframe. A transversely disposed bolster is received within each of the bolster openings and the ends of the bolster are supported by spring groups which are supported by the lower tension member of each respective sideframe.
Three piece trucks are well known for their strength, durability, and capability to support great vertical truck loads. However, a problem facing the railroad industry is that the American Association of Railroads (AAR) has set standards and established recognized practices for only discrete payload weight limits. By AAR standard M-203-83, for railcar sideframe specifications, a railroad owner/operator must choose to operate his fleet with either the AAR approved sideframe having the 6.5 inch by 12 inch journal bearing, or the 7 inch by 12 inch bearing. The former provides 100 tons of capacity per railcar and a total rail load weight of 263,000 pounds, while the latter provides 125 tons of capacity per car and a total rail load of 315,000 pounds; total rail load weight includes the payload and the weight of the train components. This also means that all railcars operating at either weight limit must meet the AAR Section 4 and 6 static and dynamic loading requirements at these two service limits. With modern day railroad operations, it is desirable to maximize the payload weight carried per mile in order to efficiently operate and contain costs. However, railroad owner/operators have found that when operating with the very large, 125 ton service loads, the rails and wheels are placed under extreme service conditions, causing them to wear in a rather short period of time. Shorter useful operating lives of the wheels and rail components is not cost feasible considering the miles of track and the number of railcars in service.
Nevertheless, owner/operators find it desirable to operate their fleets above the 100 ton standard and with systems which will be safe and cost effective. However, the AAR has only approved and standardized the 100 ton and 125 ton trucks. In order to currently operate somewhere between the 100 ton and 125 ton standards, an owner/operator is faced with a common dilemma; settle on using the smaller 100 ton trucks, or use 125 ton trucks and incur extra weight and costs for using an oversized truck.
Using the 125 ton truck and associated equipment for only 110 tons of payload capacity has not been well received in the industry since the 125 ton truck and associated equipment is very much larger and heavier and also more expensive to purchase and maintain, compared to the 100 ton truck. The added weight and expense of using a 125 ton truck in this application incrementally adds more cost per mile than can be justified by the incremental increase in payload weight gained per mile.
It is therefore the desire of the railroad owner/operators to operate with service loads of 110 tons per truck (286,000 pounds of total rail load) on trucks which are the same size and weight as the 100 ton trucks and are specifically designed to carry the 110 tons of payload.
However, an operating weakness of all trucks, and especially 100 ton trucks designed for adaptation to 110 ton service, is their tendency to be prone to fatigue cracking brought about by load cycling and to a lesser extent, static loading deflection. It should be understood that the AAR standards for dynamic loading allow the appearance of crack formations at a certain minimum number of flexure cycles as long as the sideframe can still safely operate out to the required maximum number of flexure cycles. Therefore, it should not be implyed that crack formations automatically result in catastrophic sideframe failure.
More specifically, it has been found that when adapting the standard 100 ton truck for pro-rated 110 ton payloads, and then performing the equivalent AAR static and dynamic loading performance standards on the sideframe as one would for a 100 ton loaded truck, the lower tension member of the truck sideframe is substantially susceptible to fatigue cracking, while the upper compression member is vulnerable to problems associated with increased static loading. The static loading problems are usually the result of increased vertical deflection, or reaching and/or exceeding elastic and ultimate loading limits so that failures can occur. Not particular to only the 100 ton sideframe, the area on the upper compression member, generally from the support columns to the pedestal jaws, has been cast with a reduced dimensional thickness. This has typically been done this way since the static moments closer to the jaw area are lower than the other areas of the sideframe. This means that when the 100 ton trucks are statically loaded with 110 ton payloads, the area which generally reduces in thickness, herein referred to as the transitional zone, is succeptable to stress accumulations as a result of the rather abrupt dimensional change in cross-sectional thickness, thereby weakening the sideframe. It has also been discovered that part of the stress concentration problem results after casting and is caused by the thinner cross-sectional area cooling at a faster rate than the thicker cross-sectional area. Likewise, the uneven cooling rates cause uneven shrinkage rates, and it is the uneven shrinkage rates which create the inherent internal stresses which are the result of uneven metallurgical grain structure formations. The stress accumulation is especially pronounced if there are any casting flaws present, such as internal shrinkage. In any event, the abrupt reduction in cross-sectional area will tend to concentrate the stresses and statically weaken the sideframe.
The second area on the 100 ton sideframe which experiences load-influenced problems during 110 tons of service load, is found on the lower sideframe tension member. More specifically, flexure fatigue cracking will occur on each of the upwardly extending diagonal arms, generally on the upper portion of each of the core support holes located in the arms. Since it is well known by engineering principals that stresses tend to concentrate around holes, a bending moment diagram and analysis was performed for the sideframe. It was discovered that when the dynamic flexure moments caused by 110 tons of payload were divided by the corresponding section modulus at any particular point of loading, the ratios showed that the core support hole area was substantially the weakest area on the sideframe, even though the magnitude of the flexure moments was almost the lowest.